WMM Parameters

WMM Parameters for AP,STA in 11b,11a,11g,11n mode

	CWmin	CWMax	AIFSNx	TXOP(ms)
AC_BK	15	1023	7	0
AC_BE	15	1023(63 for AP)	3	0
AC_VI	7	15	2(1 for AP)	3.008(6.016 for 11b)
AC_VO	3	7	2(1 for AP)	1.504(3.264 for 11b)

802.11 basics

DIFS

The IEEE 802.11 family of standards describe the DCF protocol, which controls access to the physical medium. A station must sense the status of the wireless medium before transmitting. If it finds that the medium is continuously idle for DCF Interframe Space (DIFS) duration, it is then permitted to transmit a frame. If the channel is found busy during the DIFS interval, the station should defer its transmission.
DIFS duration can be calculated by the following method.


DIFS = SIFS + (2 * Slot time)

Standard	Slot time (µs)	DIFS (µs)	SIFS (µs)
IEEE 802.11b	20	50	10
IEEE 802.11a	9	34	16
IEEE 802.11g	9 or 20	28 or 50	10
IEEE 802.11n (2.4 GHz)	9 or 20	28 or 50	10
IEEE 802.11n (5 GHz)	9	34	16
IEEE 802.11ac (5 GHz)	9	34	16

IEEE 802.11g is backward compatible with IEEE 802.11b. When these devices are associated with same AP all the timing parameters are changed to 802.11b.


Short Interframe Space (SIFS), is the amount of time in micro seconds required for a wireless interface to process a received frame and to respond with a response frame. It is the difference in time between the first symbol of the response frame in the air and the last symbol of the received frame in the air. A SIFS time consists of the delay in receiver RF, PLCP delay and the MAC processing delay, which depends on the physical layer used. In IEEE 802.11 networks, SIFS is the interframe spacing prior to transmission of an acknowledgment, a Clear To Send (CTS) frame, a block ack frame that is an immediate response to either a block ack request frame or an A-MPDU, the second or subsequent MPDU of a fragment burst, a station responding to any polling a by point coordination function and during contention free periods of point coordination function.

Channels

2.4 GHz (802.11b/g/n)

Graphical representation of 2.4 GHz band channels overlapping

Graphical representation of Wireless LAN channels in 2.4 GHz band

There are 14 channels designated in the 2.4 GHz range spaced 5 MHz apart (with the exception of a 12 MHz spacing before channel 14).
Note that for 802.11g/n it is not possible to guarantee orthogonal frequency-division multiplexing (OFDM) operation thus affecting the number of possible non-overlapping channels depending on radio operation.

Channel	Frequency (MHz)	United States & Canada	Europe	Switzerland	Japan		Singapore	China	Israel	Korea	Turkey	Australia	South Africa	Brazil	Taiwan	New Zealand
		40/20 MHz	40/20 MHz	Unknown MHz	40/20 MHz	10 MHz	40/20 MHz	40/20 MHz	20 MHz	20 MHz	40/20 MHz	40/20 MHz	40/20 MHz	40/20 MHz	40/20 MHz	40/20 MHz
183	4915	No	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No
184	4920	No	No	No	Yes	Yes	No	No	No	No	No	No	No	No	No	No
185	4925	No	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No
187	4935	No	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No
188	4940	No	No	No	Yes	Yes	No	No	No	No	No	No	No	No	No	No
189	4945	No	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No
192	4960	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No	No
196	4980	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No	No
7	5035	No	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No
8	5040	No	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No
9	5045	No	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No
11	5055	No	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No
12	5060	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No
16	5080	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No
34	5170	No	No	Indoors	Client only [clarification needed]	No	Yes	No	Yes	Yes	Indoors	No	Indoors	Indoors	No	Indoors
36	5180	Indoors	Yes	Indoors	Indoors	No	Yes	Yes	Yes	Yes	Indoors	Yes	Indoors	Indoors	No	Indoors
38	5190	No	No	Indoors	Client only	No	Yes	Yes	Yes	Yes	Indoors	No	Indoors	Indoors	No	Indoors
40	5200	Indoors	Yes	Indoors	Indoors	No	Yes	Yes	Yes	Yes	Indoors	Yes	Indoors	Indoors	No	Indoors
42	5210	No	No	Indoors	Client only	No	Yes	Yes	Yes	Yes	Indoors	No	Indoors	Indoors	No	Indoors
44	5220	Indoors	Yes	Indoors	Indoors	No	Yes	Yes	Yes	Yes	Indoors	Yes	Indoors	Indoors	No	Indoors
46	5230	No	No	Indoors	Client only	No	Yes	Yes	Yes	Yes	Indoors	No	Indoors	Indoors	No	Indoors
48	5240	Indoors	Yes	Indoors	Indoors	No	Yes	Yes	Yes	Yes	Indoors	Yes	Indoors	Indoors	No	Indoors
52	5260	DFS	Indoors/DFS/TPC	Indoors/DFS/TPC (otherwise limited to 100mW instead of 200mW)	Indoors/DFS/TPC	No	Yes	DFS/TPC	Yes	Yes	Indoors	DFS/TPC	Indoors	Indoors	No	DFS/TPC
56	5280	DFS	Indoors/DFS/TPC	Indoors/DFS/TPC (otherwise limited to 100mW instead of 200mW)	Indoors/DFS/TPC	No	Yes	DFS/TPC	Yes	Yes	Indoors	DFS/TPC	Indoors	Indoors	Yes	DFS/TPC
60	5300	DFS	Indoors/DFS/TPC	Indoors/DFS/TPC (otherwise limited to 100mW instead of 200mW)	Indoors/DFS/TPC	No	Yes	DFS/TPC	Yes	Yes	Indoors	DFS/TPC	Indoors	Indoors	Yes	DFS/TPC
64	5320	DFS	Indoors/DFS/TPC	Indoors/DFS/TPC (otherwise limited to 100mW instead of 200mW)	Indoors/DFS/TPC	No	Yes	DFS/TPC	Yes	Yes	Indoors	DFS/TPC	Indoors	Indoors	Yes	DFS/TPC
100	5500	DFS	DFS/TPC	DFS/TPC (otherwise limited to 500mW instead of 1W)	DFS/TPC	No	No	No	No	Yes	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
104	5520	DFS	DFS/TPC	DFS/TPC (otherwise limited to 500mW instead of 1W)	DFS/TPC	No	No	No	No	Yes	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
108	5540	DFS	DFS/TPC	DFS/TPC (otherwise limited to 500mW instead of 1W)	DFS/TPC	No	No	No	No	Yes	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
112	5560	DFS	DFS/TPC	DFS/TPC (otherwise limited to 500mW instead of 1W)	DFS/TPC	No	No	No	No	Yes	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
116	5580	DFS	DFS/TPC	DFS/TPC (otherwise limited to 500mW instead of 1W)	DFS/TPC	No	No	No	No	Yes	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
120	5600	No	DFS/TPC	DFS/TPC (otherwise limited to 500mW instead of 1W)	DFS/TPC	No	No	No	No	Yes	DFS/TPC	No	Yes	DFS	Yes	DFS/TPC
124	5620	No	DFS/TPC	DFS/TPC (otherwise limited to 500mW instead of 1W)	DFS/TPC	No	No	No	No	Yes	DFS/TPC	No	Yes	DFS	Yes	DFS/TPC
128	5640	No	DFS/TPC	DFS/TPC (otherwise limited to 500mW instead of 1W)	DFS/TPC	No	No	No	No	Yes	DFS/TPC	No	Yes	DFS	Yes	DFS/TPC
132	5660	DFS	DFS/TPC	DFS/TPC (otherwise limited to 500mW instead of 1W)	DFS/TPC	No	No	No	No	No	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
136	5680	DFS	DFS/TPC	DFS/TPC (otherwise limited to 500mW instead of 1W)	DFS/TPC	No	No	No	No	No	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
140	5700	DFS	DFS/TPC	DFS/TPC (otherwise limited to 500mW instead of 1W)	DFS/TPC	No	No	No	No	No	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
149	5745	Yes	in study, SRD (25 mW)	No	No	No	Yes	Yes	No	Yes	No	Yes	No	Yes	Yes	Yes
153	5765	Yes	in study, SRD (25 mW)	No	No	No	Yes	Yes	No	Yes	No	Yes	No	Yes	Yes	Yes
157	5785	Yes	in study, SRD (25 mW)	No	No	No	Yes	Yes	No	Yes	No	Yes	No	Yes	Yes	Yes
161	5805	Yes	in study, SRD (25 mW)	No	No	No	Yes	Yes	No	Yes	No	Yes	No	Yes	Yes	Yes
165	5825	Yes	in study, SRD (25 mW)	No	No	No	Yes	Yes	No	Yes	No	Yes	No	Yes	Yes	Yes

Wi-Fi (802.11) PHY Data Rates

Devices supporting the latest 802.11ac (draft) standard are now being certified by the Wi-Fi Alliance. 802.11ac promises various improvements over 802.11n. Starting from the first IEEE 802.11 standard in 1997 to the latest 11ac standard, there have been improvements in various aspects of 802.11 networks - speed of the network being one of the major improvements. This article attempts to explain the PHY data rates of 11b, 11a/g, 11n and 11ac.

Modulation

The above diagram shows the modulation techniques used in different 802.11 PHYsical layers. 802.11 systems support two (Ignoring the optional and obsolete types) types of modulation - DSSS/CCK and OFDM. This article is not going to cover the details of DSSS/CCK (a good intro can be found here) and OFDM. The below figure gives an overview of both the techniques.

Data Rate

DSSS/CCK (11b Date Rates)

DSSS/CCK data symbol is formed by spreading and modulating the data bits. The data rate depends on : 

• Chips per second (CSec) = 11,000,000
• Chips per symbol (CSym) = 11 (DSSS) or 8 (DSSS with CCK)
• Bits per symbol (NBits) = 1 or 2 (DSSS), 4 or 8 (DSSS with CCK)

Data Rate = (CSec/CSym)*NBits

Chips per second	Chips per symbol	Bits per symbol	Data Rate (Mbps)
11,000,000	11	1	1
11,000,000	11	2	2
11,000,000	8	4	5.5
11,000,000	8	8	11

11b Data Rates 

OFDM

DSSS/CCK uses the entire available bandwidth as one single channel. OFDM divides the channel into multiple(overlapping) sub-channels. The data rate depends on:

•  Symbol duration (in other words symbols per second) (SDur)
• Symbol duration depends on "Guard Interval" between symbols.
•  Bits per symbol (NBits)
•  Coding rate (CRate)
• Some bits are used for error correction and do not carry data 
• Number of sub-channels (NChan)
• Depends on channel width(20/40/80/160)
• 11n/11ac have more sub-channels in the same bandwidth compared to 11a/11g

Data Rate = (1/SDur)*(NBits*CRate)*NChan

11a/11g Rates

Modulation	NBits	CRate	NChan	SDur (micro sec)	Data Rate (Mbps)
BPSK	1	1/2	48	4	6
BPSK	1	3/4	48	4	9
QPSK	2	1/2	48	4	12
QPSK	2	3/4	48	4	18
16-QAM	4	1/2	48	4	24
16-QAM	4	3/4	48	4	36
64-QAM	6	2/3	48	4	48
64-QAM	6	3/4	48	4	54

11a/11g Data Rates

Note: The above table shows date rates for 20MHz channel width. 5,10 MHz channel widths are not shown.

11n/11ac Data Rates

11n and 11ac data rate improvements are due to:

• The number of sub-channels in 11n and 11ac is more than 11a and 11g. 
• Higher coding rate (5/6)
• 11ac also uses 256-QAM which further increases the data rate.
• Wider Channels (40,80,160)
• Multiple Spatial Streams (MIMO) (Nss) 

Modulation	NBits	CRate	NChan	Data Rate (Mbps) SDur=4us	Data Rate (Mbps) SDur=3.6us
BPSK	1	1/2	52	6.5	7.2
QPSK	2	1/2	52	13	14.4
QPSK	2	3/4	52	19.5	21.7
16-QAM	4	1/2	52	26	28.9
16-QAM	4	3/4	52	39	43.3
64-QAM	6	2/3	52	52	57.8
64-QAM	6	3/4	52	58.5	65
64-QAM	6	5/6	52	65	72.2
256-QAM (11ac)	8	3/4	52	78	86.7
256-QAM (11ac)	8	5/6	52	86.7	96.3

11n and 11ac Data Rates (20 MHz, 1 SS)

The entries highlighted are 11ac only rates and the rest are common to 11n and 11ac.
Updated on 20/08/2014: Note: The last entry,i.e, MCS9 (256 QAM, 5/6 rate) is not allowed in 11ac 20 MHz channel.

The maximum data rate is achieved with MIMO and maximum channel width.

Channel Width	NBits	CRate	NChan	Data Rate (Mbps) SDur=3.6us (Nss=1)	11n Max Rate (Nss=4)
20 Mhz	6	5/6	52	72.2	288.8
40 Mhz	6	5/6	108	150	600

11n Max Data Rates

Channel Width	NBits	CRate	NChan	Data Rate (Mbps) SDur=3.6us (Nss=1)	11ac Max Rate (Nss=8)
20 Mhz	8	5/6	52	96.3	577.8 (Nss=6)
40 Mhz	8	5/6	108	200	1600
80 Mhz	8	5/6	234	433.3	3466.4
160 Mhz	8	5/6	468	866.7	6933.6

11ac Max Data Rates 

Updated on 20/08/2014: Max valid spatial streams for 11ac 20 MHz channels corrected to 6.

Summary

The improvement in data rate from 11b to 11ac is due to various factors: OFDM, Wider Channels, QAM, Higher coding rate and MIMO.  The figures below depicts the improvement in data rate due to each factor(excluding MIMO).